The present invention relates to an idling speed control method and system for an engine in which a fuel injection amount and an intake air amount are simultaneously controlled to adjust an idling speed.
Conventionally, as a system for controlling the idling speed of the engine constant irrespective of disturbance of load, there has been widely employed a system as shown in FIG. 36. or as disclosed, for example, in Japanese laid-open patent application No. 60(1985)-212648.
In this conventional system, there is provided an idling speed control valve (ISCV) for regulating an idling air amount in response to a control signal from a control unit. Opening degree (opening area) of the ISCV is adjusted by adding various correction amounts according to the engine operating conditions and a feedback correction amount according to a deviation between the actual engine speed and the target idling speed to a basic characteristic value determined by an engine temperature, the feedback correction value being changed by the PI control.
In this case, an intake air amount depending upon the opening degree of the ISCV is measured by an air flow meter or an intake air pressure sensor. A fuel injection amount is determined on the basis of the measured amount of air, and an engine torque is generated according to the fuel injection amount so as to coincide the engine speed with the target idling speed.
In the conventional idling speed control, there is a problem of delay with which the change of the engine torque follows the change of the opening degree of the ISCV. That is, for example, after the opening degree of the ISCV has been increased, a delay first occurs until an increased amount of air flows into the cylinder of the engine, and then a delay occurs until such the increased amount of air is measured and a fuel amount adjusted to the increased amount of air is supplied into the cylinder. Accordingly, a considerably great delay occurs until both air and fuel are matched and the engine torque increases, which causes failing to fulfill the demand for the responsibility and the speed convergence property.
It is considered that such the delay of the engine torque increase is caused by the superposition of following four factors.
(1) Mechanical delay in response of the ISCV.
(2) Air flowing delay until an air amount induced into a cylinder becomes equal to an air amount passing through the ISCV (delay due to filling air into an intake air chamber).
(3) Delay in measuring an amount of air induced for succeeding calculation of a fuel injection amount (delay in response of a sensor for measuring an intake air amount).
(4) Fuel transport delay in transport of fuel into a cylinder due to adhesion of fuel to wall surfaces of an intake port.
If the high-speed responsive ISCV and the injector which brings less adhesion of fuel to wall surfaces are adopted, the mechanical delay (1) and the delay in transport adhered fuel (4) may be decreased. However, they cannot completely eliminate these delays (1) and (4), not to solve the air flowing delay (2) and the measurement delay (3).